It would definitely be 6 but that is not too hard considering 6 is my favorite in the series.
There were a few too many aspects of 7 that I was not really a fan of. First of all they brought back the main character that can never be removed from the party thing. I loved how you could have any and everyone in your party depending on your playstyle in 6. The limit break system was not really that amazing to me as I preferred each class or character having that extra option in combat. Things like tools, morph, runic, blitz, etc. Each character in 6 could do more than just fight and magic, they each had a unique ability that helped out the entire party. The limit break system of 7 did give each character an ability but it was just as the name says, limited. You could only do it once every several fights and then you had to build up the meter again. Plus it was the start of the very futuristic settings that came after in the series. I love a medieval fantasy setting but it seems that every FF to come after 7 was always some strange pseudo scifi future earth type world with things like cars, planes, skyscrapers, all the modern conveniences. 6 was still firmly placed in a past era even though they did manage to roll out things like magic powered mechs and helicopters. The rest of the world was still very 1600-ish in appearance.